Perfluorinated sulfonic acids (PFSAs) have excellent proton conductivity and good thermal/oxidative stability. Therefore, PFSAs are widely used in electrodialysis and chloro-alkali applications, as well as electrochemical devices such as sensors, capacitors, and fuel cells. However, PFSAs have been found to be insoluble in any single solvent, and thus are dispersive only in a mixed solvent system. Therefore, in applications such as fuel cell electrodes, water/alcohol mixtures have generally been used as PFSA dispersing agents. The current dispersion process requires a relatively high processing temperature (>200° C.), high pressure (>500 psi) and long processing time (about 18 hours) in order to ensure sufficient dispersion. In addition, the use of a closed system at this high pressure is required due to the high vapor pressure of water and/or alcohol. High temperature processing often raises concerns about degradation: crosslinking reactions, isomerization, and ether formation may occur in the acid form of perfluorinated ionomers or with alcohols. Poor dispersion or a brittle nature of the dispersion cast film may occur due to the difference in rates of evaporation between the solvents, which may be a significant problem for film fabrication or electrode performance. Typical solutions for these problems are heat treatment of membrane or electrode or adding a small amount organic polar solvent.
Hydrocarbon-based ionomers have recently emerged as suitable alternatives for electrochemical applications. This category of polymers includes sulfonated poly(arylene ether sulfone), sulfonated poly(arylene ether ketone), sulfonated poly(arylene ether nitrile), sulfonated polyphenylene and sulfonated polyimide. Hydrocarbon-based ionomers have greater thermal/oxidative stability than PFSAs and lower water transport properties. The greater thermal stability enhances the fuel cell durability under more strenuous fuel cell operating conditions. The lower water transport properties of hydrocarbon-based ionomers may be advantageous under high temperature/low relative humidity fuel cell operating conditions, as the lower water transport properties can increase water back-diffusion to the membrane and increase membrane hydration, which in turn lowers the cell resistance. Unlike PFSAs, these copolymers are amorphous in nature which prevents forming a tough film after thermal treatment when water/alcohol mixtures are used. The poor mechanical stability results in poor electrode performance, which is inferior to electrodes comprising a PFSA ionomer.
Also unlike the PFSAs, hydrocarbon-based sulfonated polymers are readily dissolved in aprotic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), N-methylpyrrolidone (NMP) or dimethylsulfoxide (DMSO). Although the hydrocarbon membranes formed with aprotic solvents are tough and ductile, solid electrodes formed from the polymer dispersion exhibited inferior performance as compared to a PFSA bonded electrode. It is believed that this inferior performance is due to the fact that electrodes made with aprotic solvents have a less porous structure, which in turn adversely impacts fuel cell performance. In addition, undesirable catalyst poisoning by the residual aprotic solvent may also occur.
Therefore, a need exists for more mechanically stable electrodes and for simpler, less costly methods of making such electrodes, which can be used to develop new electro-chemical devices. A further need exists, therefore, for solvents which are suitable for use with both PFSA and hydrocarbon-based sulfonated polymers, and which result in stable and durable fuel cells having superior performance.